1. Field of the Invention
The present invention relates to a method for avoiding a deterioration of refined edible oils and fats during transportation and/or storage thereof over a prolonged period of time.
2. Description of the Prior Art
It is generally well known that, when edible oils and fats are exposed to oxygen in the air during the storage thereof over a substantially prolonged period of time, deterioration of the quality of the edible oils and fats takes place. It is a recent trend that the refined edible oils and fats are not only circulated for domestic consumption, but also imported from or exported to foreign countries. Japan has long imported materials for the edible oils and fats for pression, extraction and refinement to produce refined edible oils and fats that can be used for domestic consumption. In recent years, however, importation of the refined edible oils and fats readily available for domestic consumption is progressively, but steadily gaining a major role because some foreign countries of origin of the material for the edible oils and fats have pursued a national policy for the material to be refined there prior to exportation thereof, or because of an attempt to minimize the transportation cost.
In any event, when the oils and fats are to be exported or imported, much tune is spent during transportation thereof from the port of dispatch to the port of destination, for example, in storing them at a wharf prior to shipment, loading them into a tanker, voyage, unloading them from the tanker at a wharf at the port of destination and storing them subsequent to the discharge from the tanker. Although the length of time spent during the transportation varies with the amount of the crude oils and fats handled, it often amounts to weeks and, in the worst case it may take about half a year subsequent to the refinement and prior to the actual use.
During the period subsequent to the refinement and prior to the actual use, the refined oils and fats may be oxidized in contact with oxygen in the air to produce peroxides or similar substances and the peroxide number of the refined oils and fats increases consequently. At the same time, the amount of free fatty acid in the refined oils and fats tends to increase, resulting in both coloring of the oils and fats and reduction in taste thereof to such an extent as to bring about a reduction in value thereof. For example, in the case of palm oil which is relatively stable because of a high content of saturated fatty acid, the storage thereof in a tank at 60.degree. C. for five days has been observed to result in an increase of the peroxide number over 1 meq/kg, accompanied by regaining the inherent color and, also, a reduction in taste. Once this happens, the palm oil is required to be refined prior to the actual use at the cost of facilities and expenditures similar to those utilized at the time of the initial refinement. In view of the foregoing, numerous attempts have long been made to avoid any possible deterioration in quality of the refined edible oils and fats during the storage and transportation.
By way of example, Japanese Laid-open Patent Publication No. -02-28505, published Mar. 3, 1977, discloses a method wherein, in order to stabilize the quality of edible oil such as, for example, soybean oil or rapeseed oil during the storage thereof, the edible oil contains a gaseous nitrogen dissolved therein in a saturated state prior to being filled in a sealed vessel such as a can. This known method does indeed contribute to minimization of the spoilage of the edible oil during the storage and transportation thereof, as the edible oil is placed in an atmosphere rich in nitrogen gas.
However, it has been found that the known method discussed above is still far from a complete avoidance of spoilage of the edible oils and fats despite the fact that the edible oils and fats are stored and transported under the atmosphere rich in nitrogen gas. By way of example, it has been observed that the storage of the edible oil under the atmosphere rich in nitrogen gas for 15 days resulted in an increase of the peroxide number over about 0.5 meq/kg, accompanied by a considerable reduction in taste and, fiance, in quality. While the above discussed method has exhibited an effective improvement over the conventional method in which the edible oils and fats are stored under the ambient atmosphere, the method is ineffective and requires an aftermath refinement immediately before the actual use particularly where storage and transportation takes a relatively long time prior to the actual use. The term "aftermath" used in conjunction with the refinement of the edible oils and fats means a refinement to be done the second time, before the actual use and after the storage for a substantial period of time, due to deterioration in quality.